1. Field of the Invention
This invention relates to an improved electron gun which is suitable for use as an electron source in an electron microscope and the like, and more particularly relates to a field emission electron gun capable of generating a highly stabilized electron beam.
2. Description of the Prior Art
As is well known, the current density of the electron beam generated by a field emission electron gun is remarkably higher than that of a thermal emission electron gun. However, in a conventional field emission electron gun, the current intensity of the electron beam is apt to fluctuate under the influence of the ion impingement on the cathode tip by the ions emanating from the anode surface due to the electron bombardment by the electrons emitted from the cathode tip.
For example, a conventional field emission electron gun as shown in FIG. 1 in cross-sectional view, includes a needle-shaped cathode tip 1 for emitting electrons and is usually made of tungsten, a hair-pin shaped filament 2 for supporting and heating the cathode tip 1 and usually made of tungsten, an anode 3 having an aperture 4 and usually made of molybdenum, a high voltage DC source 5 connected between the cathode tip 1 and the anode 3 to form an electric field for field electron emission, and a vacuum container 6.
In operating the electron gun shown in FIG. 1, the vacuum container 6 is evacuated to obtain an ultra high vacuum on the order of 10.sup.-.sup.9 .about. 10.sup.-.sup.10 Torr therein, and a high voltage of about 1.about.5 KV is applied between the cathode tip 1 and the anode 3. Then, a strong electric field of about 10.sup.7 V/cm is formed adjacent the top end of the cathode tip 1, whereby electrons are emitted therefrom. These emitted electrons are attracted by and accelerated toward the anode 3. Then, one part of the total number of emitted electrons passes through the aperture 4 and forms an electron beam e.sub.p as an effective output. However, another part of the total number of emitted electrons bombards the upper surface of the anode 3. Therefore, it there are absorbed gas molecules on the anode surface, the absorbed gas molecules are released and ionized by the electron bombardment. The ions thus emanated from the anode surface are attractively accelerated toward the cathode tip by the strong electric field and impinge on the tip at high speed. Therefore, the surface of the top end of the cathode tip is sputtered and roughened by the ion impingement, and the cathode tip is consumed little by little. Accordingly, the surface condition of the cathode tip varies, and the emission current fluctuates with time. In addition, as a higher electric field is locally applied to the protruding portions of the roughened surface of the cathode tip, electron emission therefrom gradually increases, and finally, a very large electron beam is emitted, and a vacuum discharge occurs. When such a condition occurs, the cathode tip is deformed or destroyed due to the large amount of Joule heat.
To avoid this problem, an anode heating technique has been proposed. In such a technique, the anode is heated by a suitable heating means so as to remove the absorbed gas molecules in the anode surface prior to the operation of the gun. As the anode heating means, in one case, an infrared lamp or a heater coil is provided near the anode for indirectly heating the anode. In another case, a thermal electron emitting filament is provided near the cathode tip for heating the anode by electron bombardment. Alternatively, a direct heating method, in which a heating current flows directly through the anode so as to heat the anode by Joule heat has also been proposed.
However, such conventional methods are not yet adequate for preventing the fluctuation of the emission current by reason that the absorbed gas molecules in the anode surface are not completely removed.